Add/drop node for an opical communications network

ABSTRACT

A 1550 nm DWDM optical communications network has a 1300 nm remote node added to which a small number of channels can be added or dropped. The additional node includes dual wavelength couplers to enable 1300 nm wavelength signals to be added or dropped. The 1300 nm signals are demultiplexed using a WDM filter and connected into the transponders of the 1550 nm nodes for transportation around the node. Signals for the remote network are converted to 1300 nm signals at the 1550 nm node transponders and multiplexed onto the network to be dropped to the network node transponder.

This invention relates to optical communications networks, and inparticular to add/drop nodes for adding signals to, and dropping signalsfrom, the network.

Modern optical communications networks modulate traffic using C bandwavelengths based on 1550 nm. A typical network operates at 10 Gbit withnetwork nodes spaced many kilometres apart. Photonic add/drop equipmentfor these networks is very expensive. Moreover the optical signal tonoise ratio (OSNR) degrades as additional nodes are added. Deploying alarge number of nodes in a ring network will lead to very poorperformance. Because of these dual constraints, if only a small numberof channels are required to be added or dropped at a node it may not beeconomical or even desirable from the point of view of the OSNR toinclude the node on the network.

There is, therefore, a problem in adding nodes requiring small number ofchannels, both from the point of view of OSNR and expense.

The present invention aims to overcome that problem. Broadly, theinvention provides a remote node operating with a different set ofwavelength signals in conjunction with the major add/drop nodes of thenetwork. The major nodes are used to transport add/drop traffic from theremote node around the network.

More specifically, there is provided a DWDM optical communicationsnetwork having a plurality of network nodes each for adding and droppingsignals to the network at a first set of wavelengths, and a further nodefor adding and dropping signals at a second set of wavelengths, thefurther node being arranged between adjacent nodes of the plurality ofnodes, the further node comprising a first dual wavelength coupler fordropping signals at the second set of wavelengths from the network and asecond dual wavelength coupler for adding a second set of wavelengthsonto the network, and wherein the adjacent nodes of the plurality ofnodes include transponders for transmitting signals received at thesecond set of wavelengths onto the network at the first set ofwavelengths, and receiving from the network, signals at the first set ofwavelengths to be passed to the further node at the second set ofwavelengths.

Embodiments of the invention have the advantage that a low cost remotenode can be constructed using components for a 1300 nm network which aremuch cheaper than 1550 nm DWDM network components. The remote node isonly needed to drop/add a few channels. As the wavelengths of the 1300nm signals are so far from the 1550 nm signals, any noise from theremote node signal does not affect the 1550 nm OSNR.

Preferably, the transponders of adjacent network nodes include means forconverting signals at the second set of wavelengths used by the remotenode, preferably 1300 nm, to the first set of wavelengths used by thenetwork nodes, preferably 1550 nm, and vice versa. Lower qualitynon-wavelength locked lasers may be used to convert to the second set ofwavelengths further reducing cost.

An embodiment of the invention will now be described, by way of exampleonly, and with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of a ring network including a remote add/dropnode embodying the invention;

FIG. 2 shows the remote node an adjacent network nodes in more detail;and

FIG. 3 shows the remote node transponder.

FIG. 1 illustrates a ring network 10 such as a Marconi PMA 32 opticalnetwork which carries 10 Gbit/s traffic using a 32 channel multiplexbased on a carrier wavelength of 1550 nm. A number of add/drop nodes 12to 18 are spaced at intervals of several kilometres, say about 100 kmaround the network. A remote node 20 is arranged between two of theadd/drop nodes 16, 18. The remote node can be seen as a cut down versionof the add/drop nodes 12 to 18 which operates at a different carrierwavelength, for example 1300 nm and which is invisible to the 1550 nmDWDM (dense wave division multiplex) network. The components required tobuild a 1300 nm node are very much cheaper than those required for a1550 nm node and noise on the 1300 nm node will not affect the OSNR ofthe 1550 nm network.

FIG. 2 shows the remote node in more detail. The node is arrangedbetween two main photonic add/drop nodes 16, 18 of the 1550 nm network.That network is a two fibre network with one fibre 26 carrying trafficin a West/East direction (W/E) and the other fibre 24 carrying trafficin the East/West (E/W) direction. East and West are used conventionallyand do not correspond to geographical east and west.

The remote node 20 comprises low cost 1300 nm transponders 22 whosewavelengths are such that they can be used with a coarse WDM filtersystem. The outputs from the transponders are combined and coupled ontothe main 1550 nm network fibres 24, 26 using low loss 1300/1550 nmcouplers 28, 30. For protection purposes the 1300 nm signals aretransported to the main network nodes 16, 18 where the signal isdemultiplexed using a further 1300/1550 nm coupler 29. Using anothercoarse WDM filter, shown as demultiplexer 38 the 1300 nm compositesignal is demultiplexed back into its individual channels which areconnected into the transponders 41 of the main network node via 1300 nmtransponders 22 for transportation around the ring. Signals dropped bythe main network node undergo the reverse procedure with signals fromthe 1500 nm transponders being passed to 1300 nm transponders 22,multiplexed by a 1300 nm signal multiplexer 36 and coupled onto thefibres 24, 26 using a further coupler 35.

Thus, in FIG. 2, client traffic at the remote node is received at ortransmitted from one of a pair of W/E, E/W transponders 22. Traffic tobe added to the network is multiplexed by a 1300 nm signal multiplexer36 and the 1300 nm signal multiplex is added to the network by 1300/1550nm add coupler 28 for traffic to be added to the EIW fibre 24 and by1300/1550 nm add coupler 30 for traffic to be added to the W/E fibre 26.Traffic to be dropped from the network to the transponders 22 is droppedby a 1300/1550 splitter coupler 32, 34 on each of the two fibres on thenetwork and then demultiplexed by demultiplexers 38 to restore theindividual channels which are received by the transponders 22.

Thus, each of the fibres has a pair 1300/1550 nm couplers arrangedbetween adjacent 1550 nm nodes. Traffic on the fibre between thecouplers will be mixed 1550 nm and 1300 nm traffic. However, the noisegenerated by the 1300 nm components will not affect the 1550 nm networkand so the noise budget of the network will not be affected by theremote node.

Traffic which has been placed on the network from the remote node cannotbe transported around the network as a 1300 nm signal due to fibre lossand amplifier limitations. Thus, considering the W/E path 26, traffic iscoupled onto the fibre 26 by add coupler 30. The 1300 nm traffic isdropped by splitter coupler 34, demultiplexed by the multiplexer 38 intothe individual channels. These channels are passed to the main 1550 nmnetwork node transponders where they are transported around the ringnetwork as 1550 nm signals.

FIG. 3 shows the transponder at the 1550 nm nodes required to add anddrop the 1300 nm signal to and from the network. On the add side, agrey, non wavelength specific signal is received and is converted to anelectrical signal by photo diode 50 and amplified by amplifier 52.

At 54, after retiming, the signal may be further processed to add errordetection and system management information before being re-transmittedat the required wavelength by laser 56.

On the drop side, a received signal is converted to an electrical signalby a photodiode 59 and an amplifier 60. After retiming at 62, the signalmay be further processed to detect errors and management information.The signal is converted back to an optical signal by laser 64. This is agrey laser which is inexpensive and which outputs a grey signal to theclient.

At the remote node, the lasers used in the transponders can benon-wavelength locked and directly modulated. As these nodes only use asmall number of channels, which is why a full 1550 nm node is notneeded, they may be WDM transponders instead of the dense WDM requiredon the main network. As the channels are spaced further apart on a WDMnetwork than on a DWDM network, controlling drift of the laser withtemperature may not be required, further simplifying the construction.

Thus, the embodiment described further provides a low cost remote nodefor an optical network that is suitable for use where the channelrequirement is not large enough to warrant a full node. The nodefunctions at a different wavelength to the main network so that anynoise at that wavelength introduced onto the network does not affect theOSNR of the main network.

Various modifications to the embodiment described are possible and willoccur to those skilled in the art without departing from the inventionwhich is defined by the following claims.

1-11. (canceled)
 12. A dense wavelength division multiplexing (DWDM)optical communications network, comprising: a plurality of network nodeseach for adding and dropping signals to the network at a first set ofwavelengths; a further node for adding and dropping signals at a secondset of wavelengths not included in the first set of wavelengths, thefurther node being arranged between adjacent nodes of the plurality ofnodes, the further node comprising a first dual wavelength coupler fordropping signals at the second set of wavelengths from the network, anda second dual wavelength coupler for adding the second set ofwavelengths onto the network; and the adjacent nodes of the plurality ofnodes including transponders for transmitting signals received at thesecond set of wavelengths onto the network at the first set ofwavelengths, and for receiving from the network, signals at the firstset of wavelengths to be passed to the further node at the second set ofwavelengths.
 13. The DWDM optical communications network according toclaim 12, wherein the network between the dual wavelength couplerscarries signals at the first and second set of wavelengths.
 14. The DWDMoptical communications network according to claim 12, wherein thefurther node comprises means for splitting a signal multiplex receivedfrom the network in a plurality of separate channels each at one of thesecond set of wavelengths.
 15. The DWDM optical communications networkaccording to claim 14, wherein the means for splitting comprises anoptical demultiplexer.
 16. The DWDM optical communications networkaccording to claim 14, wherein the further node comprises means forcombining signal channels at individual wavelengths of the second set ofwavelengths for addition onto the network.
 17. The DWDM opticalcommunications network according to claim 16, wherein the combiningmeans comprises a multiplexer.
 18. The DWDM optical communicationsnetwork according to claim 12, wherein the transponders at the adjacentnodes of the plurality of network nodes comprises means for convertingtraffic received at the second set of wavelengths into an electricalsignal, and means for converting the electrical signal to an opticalsignal at one of the first set of wavelengths.
 19. The DWDM opticalcommunications network according to claim 12, wherein the transpondersat the adjacent nodes of the plurality of network nodes comprises meansfor bandpass filtering traffic received from the network to select asingle channel, means for converting the selected channel into anelectrical signal, and means for converting the electrical signal to agrey optical signal at the second set of wavelengths.
 20. The DWDMoptical communications network according to claim 12, wherein thefurther node comprises a transponder comprising means for convertingtraffic for addition to the network at the second set of wavelengthsinto an electrical signal, and means for converting the electricalsignal as a wavelength division multiplexing (WDM) signal at the secondset of wavelengths.
 21. The DWDM optical communications networkaccording to claim 12, wherein the first set of wavelengths is based on1550 nm, and wherein the second set of wavelengths is based on 1300 nm.22. A 1300 nm wavelength WDWM node for use with a 1550 nm wavelengthdense wavelength division multiplexing (DWDM) optical communicationsnetwork, the WDWM node comprising: a pair of 1300/1550 nm couplers foradding and dropping 1300 nm signals to and from a path of the network; a1300 nm multiplexer for multiplexing channels of wavelength divisionmultiplexing (WDM) signal for addition to the network; and ademultiplexer for demultiplexing 1300 nm signals from the network toprovide a plurality of WDM channels.